Casting roll for a two-roll continuous casting installation

ABSTRACT

A casting roll of a two-roll continuous casting installation is to be able to be exposed to a changing temperature stress and roll pressures when casting strip made of non-ferrous metals, especially of aluminum or an aluminum alloy. For this purpose, its sleeve is made of an age-hardening copper alloy made of—as expressed in each case as weight %—0.4% through 2% cobalt, which is partially exchangeable for nickel, 0.1% through 0.5% beryllium, optionally 0.03% through 0.5% zirconium, 0.005% through 0.1% magnesium and possibly a maximum of 0.15% of at least one element of the group including niobium, manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium, the remainder being copper and inclusive of manufacturing-conditioned impurities and usual processing additives.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a casting roll for a two-roll continuouscasting installation.

[0003] 2. Description of Related Art

[0004] The worldwide aim, especially of the steel industry, to poursemifinished product as close to final dimensions as possible, in orderto save hot and/or cold working steps, has led since about 1980 to aseries of developments, such as single roll and two-roll continuouscasting methods.

[0005] In these casting methods, very high surface temperatures appearat the water-cooled cylinders or rolls during casting of steel alloys,nickel, copper, as well as alloys that are only rolled with difficultyin the pouring range of the melt. In the case of close to finaldimension casting of a steel alloy, for example, the temperatures areabout 350° C. to 450° C., the sleeves of the continuous casting rollsbeing made of a CuCrZr material having an electrical conductivity of 48Sm/mm² and a heat conductivity of about 320 W/mK. Materials based onCuCrZr were used up to now predominantly for continuous casting dies andcasting wheels that were thermally highly stressed. In the case of thesematerials, the surface temperature drops cyclically to about 150° C. to200° C., by the cooling of the casting rolls, with each revolution,shortly before the casting range. On the cooled rear side of the castingrolls, however, the temperature remains largely constant during thecycle, at about 30° C. to 40° C. The temperature gradient between thesurface and the rear side in combination with the cyclical change in thesurface temperature of the continuous casting rolls causes thermalstress in the surface region of the sleeve material.

[0006] According to investigations of the fatigue properties of theCuCrZr materials used up to now, at various temperatures, using anexpansion amplitude of +/−0.3% and a frequency of 0.5 Hertz—theseparameters approximately correspond to a rotational speed of thecontinuous casting rolls of 30 rpm—one may expect, for example, in thefavorable case, a service life of 3000 cycles until cracks form, using amaximum surface temperature of 400° C., corresponding to a wallthickness of 25 mm above the water cooling. Therefore, the continuouscasting rolls have to be reconditioned after as relatively early anoperating time as about 100 minutes, for the purpose of removing surfacecracks. In this context, the service life between reworking is, amongother things, substantially dependent on the effectiveness of thelubrication/release agents at the casting surface, the constructive andprocess-conditioned cooling as well as the casting speed. For thepurpose of exchanging the continuous casting rolls, the castinginstallation has to be stopped and the casting process has to beinterrupted.

[0007] A further disadvantage of the proven mold material CuCrZr forthis particular application is the relatively low hardness ofapproximately 110 HBW to 130 HBW. For, in the case of a single roll ortwo-roll continuous casting method, it is not to be avoided that, evenbefore the casting range, splashes appear on the roll surfaces. Thesolidified steel particles are then pressed into the relatively softsurfaces of the continuous casting rolls, whereby the surface quality ofthe poured strips of about 1.5 mm to 4 mm thickness are considerablyimpaired.

[0008] Compared to a CuCrZr alloy, the lower electrical conductivity ofa known CuNiBe alloy, having an addition of up to 1% niobium, also leadsto a higher surface temperature. Since the electrical conductivitybehaves approximately proportionally to the heat conductivity, thesurface temperature in the sleeve, of a continuous casting roll, made ofthe CuNiBe alloy as compared to a continuous casting roll having asleeve made of CuCrZr, at a maximum temperature of 400° C. at thesurface and 30° C. on the rear side will be increased to about 540° C.

[0009] Ternary CuNiBe and CuCoBe alloys do indeed basically demonstratea Brinell hardness of more than 200 HBW, however, the electricalconductivity of the standard semifinished products made of thesematerials, such as rod for manufacturing resistance welding electrodesor sheet or strip for manufacturing springs or leadframes, reach valuesof at most in the range of 26 Sm/mm² to about 32 Sm/mm². Under optimumconditions, with the use of these standard materials, a surfacetemperature of only about 585° C. could be reached at the sleeve of acontinuous casting roll.

[0010] Even from the CuCoBeZr and CuNiBeZr alloys basically known fromU.S. Pat. No. 4,179,314, no hints are seen that conductivity valuesof >38 Sm/mm² in conjunction with a minimum hardness of 200 HBW could beachieved.

[0011] Within the scope of EP 0 548 636 B1, the use of an age-hardeningcopper alloy is also related art, which has 1.0% to 2.6% nickel that maybe fully or partially replaced by cobalt, 0.1% to 0.45% beryllium,optionally 0.05% to 0.25% zirconium and possibly up to a maximum of0.15% of at least one of the group of elements including niobium,tantalum, vanadium, titanium, chromium, cerium and hafnium, the restbeing copper inclusive of production-conditioned contaminations andusual processing additives, having a Brinell hardness of at least 200HBW and an electrical conductivity greater than 38 Sm/mm² as thematerial for producing continuous casting rolls and wheels.

[0012] Alloys having these compositions, such as the alloys CuCo2Be0.5or CuNi2Be0.5, have disadvantages in their hot forming capability,because of their relatively high alloying element content. However, highheat deformation strains are required to attain a fine grained producthaving a grain size <1.5 mm (as per ASTM E 112), starting from acoarse-grained cast structure having a grain size of severalmillimeters. In particular, for large format casting rolls, up to thispoint, sufficiently large continuous casting rolls have been producibleonly at very high expenditure; however, technical shaping devices arehardly available for realizing, at a justifiable cost, a sufficientlyhigh hot kneading for recrystallization of the cast structure into afine grain structure.

SUMMARY OF THE INVENTION

[0013] It is an object of the invention to create a continuous castingroll as a component of a two-roll continuous casting installation,which, during close to final dimension casting of strips made ofnon-ferrous metals, may be exposed without any problem to changingtemperature stresses and high roll pressures, while having a longservice life.

[0014] These and other objects of the invention are attained by acasting roll for a two-roll continuous casting installation, which has asleeve made of an age-hardening copper alloy made of—as expressed ineach case as weight %—0.4% through 2% cobalt, which is partiallyexchangeable for nickel, 0.1% through 0.5% beryllium, optionally 0.03%through 0.5% zirconium, 0.005% through 0.1% magnesium and possibly amaximum of 0.15% of at least one element of the group including niobium,manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium,the remainder being copper inclusive of manufacturing-conditionedimpurities and usual processing additives. During the casting of stripsmade of non-ferrous metals, the casting roll may undergo changingtemperature stress and high roll pressures.

DETAILED DESCRIPTION OF THE INVENTION

[0015] By the use of a CuCoBeZr(Mg) alloy having an intentionally gradedlow content of Co and Be, on the one hand, one may ensure a stillsufficient age-hardenability of the material for achieving highstrength, hardness and conductivity; on the other hand, only low heatdeformation strain is required for the complete recrystallization of thecast structure and the setting of a fine-grained structure havingsufficient ductility.

[0016] Due to a continuous casting roll thus developed as a component ofa two-roll continuous casting installation, it is possible to increasethe casting speed of a strip made of a non-ferrous metal, particularlyof aluminum or an aluminum alloy, by more than double, compared to aroll installation in which the rolls are fitted with steel sleeves. Inaddition, a clearly improved surface quality of the cast strip isachieved. Also, a considerably longer service life is ensured for thesleeve.

[0017] This continuous casting roll may be developed as a hollowcylinder, i.e. inherently rigid without a core. The surface coming intocontact with the strips to be cast, however, may also be a component ofa sleeve having a core, especially a steel core. The sleeve may then beshrink fitted onto such a core as the carrier, hot isostatically pressedon or slipped on and then locked mechanically.

[0018] It is also conceivable, when using a sleeve, that this could bedeveloped as a single layer or multiple layers.

[0019] The enveloping surface of the surface of the casting roll may bedesigned cylindrically or having a camber, so as possibly to compensatefor the sagging of a roll.

[0020] A further improvement in the sleeve's mechanical properties,particularly an increase in tensile strength, may be advantageouslyachieved, if the copper alloy contains 0.03% to 0.35% zirconium, and0.005% to 0.05% magnesium.

[0021] According to a further specific embodiment, the copper alloycontains a proportion <1.0% of cobalt, 0.15% to 0.3% of beryllium and0.15% to 0.3% of zirconium.

[0022] It is also of advantage if the ratio of cobalt to beryllium inthe copper alloy of the sleeve is between 2 and 15. Most preferably,this ratio of cobalt to beryllium is 2.2 to 5.

[0023] The invention permits having the copper alloy contain, inaddition to cobalt, up to 0.6% nickel.

[0024] Further improvements of the mechanical properties of the castingroll may be achieved if the copper alloy of the sleeve contains up to amaximum of 0.15% of at least one element of the group including niobium,manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium.

[0025] The sleeve is advantageously produced by the processing stepscasting, hot working, solution treatment at 850° C. to 980° C., coldworking up to 30% as well as age-hardening at 400° C. to 550° C. withina period of 4 to 32 hours, the sleeve having a maximum average grainsize of 1.5 mm as per ASTM E 112, a hardness of at least 170 HBW, and anelectrical conductivity of at least 26 Sm/mm².

[0026] It is of particular advantage if the sleeve in the age-hardenedstate, has an average grain size of 30 μm to 500 μm as per ASTM E 112, ahardness of at least 185 HBW, a conductivity between 30 and 36 Sm/mm², a0.2% yield strength of at least 450 MPa and an elongation at break of atleast 12%.

[0027] If the sleeve is provided with a coating which reduces thepermeability to heat, or evens out the flow of heat, the product qualityof the cast strip made of a non-ferrous metal, but particularly ofaluminum or an aluminum alloy, is further enhanced. Based on theoperating condition of the sleeve, this coating, specifically made of acopper alloy, is made effective, especially in the case of an aluminumstrip, due to the fact that, at the beginning of a casting or rollingprocess, an adhesion layer forms from the acting together of copper andaluminum on the surface of the sleeve, from which, then, during thefurther course of the casting process aluminum penetrates the coppersurface and there forms a stable, resistive diffusion layer, whosethickness and properties are essentially determined by the casting speedand cooling conditions. That clearly improves the surface quality of thealuminum strip and consequently the product quality.

[0028] The service life of the sleeve can be prolonged even further byusing a coating having a great surface hardness.

[0029] The surface of the casting roll may be made smooth. This designis achievable particularly by rolling. In this manner, pressure stressesare induced in the edge zone, and these make possible additionalresistance to the formation of cracks and the progression of cracks, soas to raise the life duration of the casting roll.

[0030] The surface of the casting roll may be textured. Texturing can beapplied, for example, by cutting, roller-burnishing, eroding orblasting. With the use of such measures, the heat transfer coefficientmay specifically be influenced.

[0031] In the depressions formed by the texturing, a substance may beembedded having a low heat conductivity compared to the heatconductivity of copper.

[0032] Besides being a metallic material, such as particularly nickel ora nickel alloy, such a substance may also be a ceramic material. Such afilling up of the depressions formed by the texturing on the surface ofthe casting roll is used to create good surface quality and to ensure alasting influence on the heat conductivity.

[0033] The invention is explained in greater detail with reference tothe examples below. In the light of seven alloys for the sleeve of acasting roll (alloys A to G) and three comparison alloys (H to J), it isshown how critical the composition is to achieving the combinations ofproperties aimed for.

[0034] All the alloys were smelted in a crucible furnace and cast intoround billets of equal format. The composition of the individual layersis given below in Table 1. The addition of magnesium is made for thepre-deoxidization of the melt, and the addition of zirconium actspositively on the hot ductility. TABLE 1 Alloy Co (%) Ni (%) Be (%) Zr(%) Mg (%) Cu (%) A 0.68 — 0.20 0.20 0.03 Rest B 1.0 — 0.22 0.22 0.03Rest C 1.4 — 0.20 0.18 0.02 Rest D 0.65 — 0.29 0.21 0.04 Rest E 1.0 —0.31 0.24 0.01 Rest F 1.4 — 0.28 0.19 0.03 Rest G 1.0 0.1 0.22 0.16 0.03Rest H — 1.7 0.27 0.16 — Rest 1 2.1 — 0.55 0.24 — Rest J — 1.4 0.54 0.20— Rest

[0035] The alloys were subsequently pressed into flat bars using a lowpressure ratio (=cross section of the cast block/cross section of thepressed bar) of 5.6:1 on an extrusion press at 950° C. Thereafter, thealloys were submitted to an at least 30-minute solution treatment above850° C., using a subsequent water quenching, and after that, wereage-hardened for 4 to 32 hours at a temperature range between 400° C.and 550° C. The combinations of properties attained are shown in Table 2below. TABLE 2 Rm Rp_(0.2) A HBW 2.5 El. Cond. Grain Size Alloy MPa MPa% 187.5 Sm/mm² mm A 694 492 21 207 36.8 0.09-0.25 B 675 486 18 207 32.80.09-0.18 C 651 495 18 211 30.0 0.045-0.13  D 707 501 19 207 31.40.09-0.25 E 735 505 19 229 33.6 0.045-0.18  F 735 520 19 224 32.30.09-0.25 G 696 513 18 213 33.5 0.065-0.18  H 688 556 10 202 41.0 2-3 1784 541 11 229 30.3 1.5-3   J 645 510  4 198 30.9 4-6

[0036] As may be seen from the combinations of properties, the alloysaccording to the present invention, for producing a sleeve of a castingroll, attain the aimed-for recrystallized fine grained structure whilehaving an appropriately good elongation at break. In the case ofcomparison alloys H to J, there is a grain size of more than 1.5 mm,which reduces the ductility of the material.

[0037] An additional increase in strength may be attained by coldforming before the age-hardening. Table 3 below gives the propertycombinations of alloys A to J, which are achieved by solution treatmentof the pressed material for at least 30 minutes above 850° C. andsubsequent water quenching, 10% to 15% cold rolling (reduction in crosssection) and then age-hardening from 2 to 32 hours at a temperaturerange between 400° C. and 550° C. TABLE 3 Rm Rp₀ ₂ A HBW 2.5 El. Cond.Grain Size Alloy MPa MPa % 187.5 Sm/mm² mm A 688 532 20 211 36.7 0.13-0.25 B 679 534 18 207 34.6 0.045-0.18 C 741 600 17 227 34.40.065-0.18 D 690 537 21 207 32.6 0.065-0.25 E 735 576 19 230 34.70.045-0.18 F 741 600 17 227 34.4  0.13-0.25 G 695 591 15 224 33.0 0.18-0.35 H 751 689  9 202 40.9  2-4 1 836 712 10 229 31.0  2-3 J 726651  6 198 31.5  3-6

[0038] Alloys A to G according to the present invention, in turn,demonstrate good elongations at break and a grain size less than 0.5 mm,while comparison alloys H to J have a coarse grain, having a grain sizegreater than 1.5 mm and lower values of elongation at break. Thus, thesecopper alloys have clear processing advantages during the production ofsleeves, particularly for larger continuous casting rolls of two-rollcasting installations, whereby it is made possible to produce a finegrained end product having optimum basic properties for their field ofapplication.

What is claimed is:
 1. A casting roll for a two-roll continuous castinginstallation which, during the casting of strips made of non-ferrousmetals, undergoes changing temperature stress and high roll pressures,comprising a sleeve made of an age-hardening copper alloy, whichincludes in weight %: 0.4% through 2% cobalt, which may be partiallysubstituted with nickel, 0.1% through 0.5% beryllium, and a remainder ofcopper.
 2. The casting roll according to claim 1, in which the copperalloy further includes 0.03% through 0.5% zirconium and 0.005% through0.1% magnesium.
 3. The casting roll according to claim 1, in which thecopper alloy further includes a maximum of 0.15% of at least one elementselected from the group consisting of niobium, manganese, tantalum,vanadium, titanium, chromium, cerium and hafnium.
 4. The casting rollaccording to claim 2, in which the copper alloy contains 0.03% to 0.35%zirconium and 0.005% to 0.05% magnesium.
 5. The casting roll accordingto claim 2, in which the copper alloy contains less than 1.0% cobalt,0.15% through 0.3% beryllium and 0.15% through 0.3% of zirconium.
 6. Thecasting roll according to claim 1, in which the copper alloy has a ratioof cobalt to beryllium of between 2 and
 15. 7. The casting rollaccording to claim 2, in which the copper alloy has a ratio of cobalt toberyllium of between 2 and
 15. 8. The casting roll according to claim 3,in which the copper alloy has a ratio of cobalt to beryllium of between2 and
 15. 9. The casting roll according to claim 6, in which the copperalloy has a ratio of cobalt to beryllium of between 2.2 and
 5. 10. Thecasting roll according to claim 1, in which the copper alloy furtherincludes up to 0.6% nickel in addition to cobalt.
 11. The casting rollaccording to claim 2, in which the copper alloy further includes up to0.6% nickel in addition to cobalt.
 12. The casting roll according toclaim 2, in which the copper alloy further includes a maximum of 0.15%of at least one element selected from the group consisting of niobium,manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium.13. The casting roll according to claim 1, wherein the sleeve isproduced by casting, hot working, solution treatment at 850° C. to 980°C., cold working up to 30% as well as age-hardening at 400° C. to 550°C. within a time period of 4 to 32 hours, the sleeve having a maximumaverage grain size of 1.5 mm as per ASTM E 112, a hardness of at least170 HBW, and an electrical conductivity of at least 26 Sm/mm².
 14. Thecasting roll according to claim 13, in which the sleeve, in theage-hardened state, has an average grain size of 30 μm to 500 μm as perASTM E 112, a hardness of at least 185 HBW, a conductivity between 30and 36 Sm/mm², a 0.2% yield strength of at least 450 MPa and anelongation at break of at least 12%.
 15. The casting roll according toclaim 1, in which the sleeve is provided with a coating that reduces thepermeability to heat.
 16. The casting roll according to claim 15, inwhich the coating has a great surface hardness.
 17. The casting rollaccording to claim 1, in which the surface is designed to be smooth. 18.The casting roll according to claim 1, in which the surface is textured.19. The casting roll according to claim 18, in which a substance isembedded in the depressions, formed by the texturing, which has a lowheat conductivity compared to the heat conductivity of copper.